Only a portion of oil present in an oil-bearing formation is recoverable as a result of the natural pressure of the formation. The oil recovered from this “primary” recovery typically ranges from 5% to 35% of the oil in the formation. Enhanced oil recovery methods have been developed to increase the amount of oil that may be recovered from an oil-bearing formation above and beyond that recovered in primary recovery.
Water-flooding, in which water is injected through an injection well into an oil-bearing formation to mobilize and drive oil through the formation for production from a production well, is a widely used method of secondary recovery used to increase the amount of oil recovered from a formation beyond primary recovery. The amount of oil produced by water-flooding may be reduced by water fingering through the oil in the formation due in part to viscosity differences between the injected water and oil in the formation rendering water more mobile than oil in the formation. Oil by-passed by water fingering is left in place in the formation and is typically not recovered by further water-flooding since additional water injected into the formation follows the path of the initial water through the formation.
Water-soluble polymer has been added to water injected into an oil-bearing formation to increase the viscosity of the water and decrease the viscosity difference between the injected water and oil in the formation, improving the water to oil mobility ratio and thereby reducing water fingering through the oil. This improves the sweep efficiency of the water in the formation and increases oil recovery. The aqueous polymer mixture may drive through the formation in a plug-like flow to mobilize the oil in the formation for production with reduced fingering of the aqueous drive solution through the oil relative to water without polymer.
Ionically charged water-soluble polymers have been utilized with low salinity water, where “low salinity” water has a total dissolved solids (hereafter “TDS”) content of 15,000 parts per million (hereafter “ppm”) or less, to produce an aqueous polymer solution for use in recovering oil from an oil-bearing formation. Use of an ionically charged water-soluble polymer with low salinity water provides a substantial viscosity increase to the water with a minimum quantity of polymer.
Fluids produced from an oil-bearing formation in which an aqueous polymer mixture is used as a water-flooding oil recovery agent contain an aqueous polymer-containing phase and an oil phase, where the aqueous polymer-containing phase has a viscosity that is comparable to the viscosity of the oil phase. The relatively high viscosity of the aqueous polymer-containing phase renders separation of the aqueous polymer-containing phase from the oil phase more difficult than separation of oil and water in the absence of a water-soluble polymer. The viscosity of the aqueous polymer-containing phase of the produced fluid may be reduced by mechanically shearing the produced fluid, however, mechanical shearing induces emulsion formation between the aqueous and oil phases of the produced fluid which inhibits clean separation of the oil and aqueous phases.
Improved processes are desirable for reducing the viscosity of polymer-containing fluids produced from an oil-bearing formation in which oil is produced from the formation utilizing a low salinity water-flood in which the low salinity water contains a water-soluble polymer.